1. Field of the Invention
This invention relates to a method and apparatus for positioning and, more particularly, to a method and GPS (Global Positioning System) receiver for positioning on the Globe by making use of satellite signals transmitted from a plurality of artificial satellites.
2. Description of the Prior Art
One of the positioning systems currently in service utilizing artificial satellites is known as a so-called Global Positioning System (GPS). In accordance with this positioning system, the entire surface of the Earth is covered by a total of 24 GPS satellites when all of them are launched in six orbits at approximately 20,200 km above the Earth with four GPS satellite each. Each GPS satellite transmits a satellite signal containing navigational data through a spread-spectrum transmission system. Positioning may be performed on the ground, on the sea or in the air by receiving satellite signals from a plurality of GPS satellites, for example, by receiving the satellite signals from three GPS satellites for two-dimensional positioning and from four GPS satellites for three-dimensional positioning. Based on the navigational data contained in the satellite signal from each GPS satellite, positional information of the receiving point such as a latitude, longitude and altitude can be determined in real time.
This Global Positioning System was originally developed for U.S. military use, however, a part of the satellite or GPS signal (C/A code) has been made available for civil use. Therefore, it becomes possible to provide a navigation system for use in motor vehicles, water vessels and aircraft by making use of the satellite signals.
Since the satellite signals are transmitted by the spread-spectrum transmission system, the reception of the satellite signal must be initiated by locking a phase-locked loop (PLL) circuit of the GPS receiver exactly on a receiving frequency of the satellite signal. Upon successful locking of the PLL circuit, the received spread-spectrum signal is despread and demodulated to receive navigation data.
One of the typical motor vehicle-mounted navigation apparatus of the prior art for use in the GPS system is a single-channel type GPS receiver having a single antenna, single RF receiving unit and single signal processing unit. Accordingly, it has been necessary to receive satellite signals from the minimum number of GPS satellites in a sequential manner through the single receiving channel for positioning.
Another type of known motor vehicle-mounted GPS receiver is a multi-channel GPS receiver having a plurality of receiving channels each of which is assigned for receiving a satellite signal from an allotted GPS satellite.
In the prior art system, in case of receiving satellite signals transmitted by a plurality of GPS satellites with a single antenna, obstructions such as a building and the like have prevented the GPS receiver from receiving the satellite signals transmitted by the predetermined minimum number of GPS satellites which are needed for positioning. This problem is particularly encountered with the motor vehicle-mounted GPS receiver which moves together with the motor vehicle.
As described, in the typical multi-channel type motor vehicle-mounted GPS receiver of the prior art, the required number of GPS satellites for positioning has been shared with multiple receiving channels for receiving satellite signals transmitted therefrom. However, one of the disadvantages of this prior art system is that, if any of the GPS satellites, a satellite signal from which has been received by one of the receiving channels of the GPS receiver, is obstructed by a building or other obstacles, it will become impossible to receive the required satellite signals from the minimum number of GPS satellites for positioning. This results in an interruption of the positioning by the GPS receiver.
It is therefore an object of the present invention to provide a GPS receiver which is capable of maintaining as good receiving condition as possible even if any GPS satellite in use is obstructed by an obstacle in the midst of positioning.
It is another object of the present invention to provide a positioning method for use with a motor vehicle-mounted GPS receiver, wherein the GPS receiver is maintained at high degree of accuracy of positioning even if any GPS satellite in use is obstructed by an obstacle and resulting in no reception of satellite signal therefrom in the midst of positioning.